20,938 research outputs found
Low thrust viscous nozzle flow fields prediction
A Navier-Stokes code was developed for low thrust viscous nozzle flow field prediction. An implicit finite volume in an arbitrary curvilinear coordinate system lower-upper (LU) scheme is used to solve the governing Navier-Stokes equations and species transportation equations. Sample calculations of carbon dioxide nozzle flow are presented to verify the validity and efficiency of this code. The computer results are in reasonable agreement with the experimental data
An Analytical Approach to Inhomogeneous Structure Formation
We develop an analytical formalism that is suitable for studying
inhomogeneous structure formation, by studying the joint statistics of dark
matter halos forming at two points. Extending the Bond et al. (1991) derivation
of the mass function of virialized halos, based on excursion sets, we derive an
approximate analytical expression for the ``bivariate'' mass function of halos
forming at two redshifts and separated by a fixed comoving Lagrangian distance.
Our approach also leads to a self-consistent expression for the nonlinear
biasing and correlation function of halos, generalizing a number of previous
results including those by Kaiser (1984) and Mo & White (1996). We compare our
approximate solutions to exact numerical results within the excursion-set
framework and find them to be consistent to within 2% over a wide range of
parameters. Our formalism can be used to study various feedback effects during
galaxy formation analytically, as well as to simply construct observable
quantities dependent on the spatial distribution of objects. A code that
implements our method is publicly available at
http://www.arcetri.astro.it/~evan/GeminiComment: 41 Pages, 11 figures, published in ApJ, 571, 585. Reference added,
Figure 2 axis relabele
Accurate determination of the Lagrangian bias for the dark matter halos
We use a new method, the cross power spectrum between the linear density
field and the halo number density field, to measure the Lagrangian bias for
dark matter halos. The method has several important advantages over the
conventional correlation function analysis. By applying this method to a set of
high-resolution simulations of 256^3 particles, we have accurately determined
the Lagrangian bias, over 4 magnitudes in halo mass, for four scale-free models
with the index n=-0.5, -1.0, -1.5 and -2.0 and three typical CDM models. Our
result for massive halos with ( is a characteristic non-linear
mass) is in very good agreement with the analytical formula of Mo & White for
the Lagrangian bias, but the analytical formula significantly underestimates
the Lagrangian clustering for the less massive halos $M < M_*. Our simulation
result however can be satisfactorily described, with an accuracy better than
15%, by the fitting formula of Jing for Eulerian bias under the assumption that
the Lagrangian clustering and the Eulerian clustering are related with a linear
mapping. It implies that it is the failure of the Press-Schechter theories for
describing the formation of small halos that leads to the inaccuracy of the Mo
& White formula for the Eulerian bias. The non-linear mapping between the
Lagrangian clustering and the Eulerian clustering, which was speculated as
another possible cause for the inaccuracy of the Mo & White formula, must at
most have a second-order effect. Our result indicates that the halo formation
model adopted by the Press-Schechter theories must be improved.Comment: Minor changes; accepted for publication in ApJ (Letters) ; 11 pages
with 2 figures include
The cosmological light-cone effect on the power spectrum of galaxies and quasars in wide-field redshift surveys
We examine observational consequences of the cosmological light-cone effect
on the power spectrum of the distribution of galaxies and quasars from upcoming
redshift surveys. First we derive an expression for the power spectrum of
cosmological objects in real space on a light cone, , which is exact in linear theory of density perturbations. Next we
incorporate corrections for the nonlinear density evolution and redshift-space
distortion in the formula in a phenomenological manner which is consistent with
recent numerical simulations. On the basis of this formula, we predict the
power spectrum of galaxies and quasars on the light cone for future redshift
surveys taking account of the selection function properly. We demonstrate that
this formula provides a reliable and useful method to compute the power
spectrum on the light cone given an evolution model of bias.Comment: 18 pages, 3 figures, to be published in the Astrophysical Journa
Evolution of Lyman Break Galaxies Beyond Redshift Four
The formation rate of luminous galaxies seems to be roughly constant from z~2
to z~4 from the recent observations of Lyman break galaxies (LBGs) (Steidel et
al 1999). The abundance of luminous quasars, on the other hand, appears to drop
off by a factor of more than twenty from z~2 to z~5 (Warren, Hewett, & Osmer
1994; Schmidt, Schneider, & Gunn 1995). The difference in evolution between
these two classes of objects in the overlapping, observed redshift range,
z=2-4, can be explained naturally, if we assume that quasar activity is
triggered by mergers of luminous LBGs and one quasar lifetime is ~10^{7-8} yrs.
If this merger scenario holds at higher redshift, for the evolutions of these
two classes of objects to be consistent at z>4, the formation rate of luminous
LBGs is expected to drop off at least as rapidly as exp(-(z-4)^{6/5}) at z>4.Comment: in press, ApJ Letters, 15 latex pages plus 1 fi
Deriving the Nonlinear Cosmological Power Spectrum and Bispectrum from Analytic Dark Matter Halo Profiles and Mass Functions
We present an analytic model for the fully nonlinear power spectrum P and
bispectrum Q of the cosmological mass density field. The model is based on
physical properties of dark matter halos, with the three main model inputs
being analytic halo density profiles, halo mass functions, and halo-halo
spatial correlations, each of which has been well studied in the literature. We
demonstrate that this new model can reproduce the power spectrum and bispectrum
computed from cosmological simulations of both an n=-2 scale-free model and a
low-density cold dark matter model. To enhance the dynamic range of these large
simulations, we use the synthetic halo replacement technique of Ma & Fry
(2000a), where the original halos with numerically softened cores are replaced
by synthetic halos of realistic density profiles. At high wavenumbers, our
model predicts a slope for the nonlinear power spectrum different from the
often-used fitting formulas in the literature based on the stable clustering
assumption. Our model also predicts a three-point amplitude Q that is scale
dependent, in contrast to the popular hierarchical clustering assumption. This
model provides a rapid way to compute the mass power spectrum and bispectrum
over all length scales where the input halo properties are valid. It also
provides a physical interpretation of the clustering properties of matter in
the universe.Comment: Final version to appear in the Astrophysical Journal 544 (2000).
Minor revisions; 1 additional figure. 25 pages with 6 inserted figure
Two Aspects of the Mott-Hubbard Transition in Cr-doped V_2O_3
The combination of bandstructure theory in the local density approximation
with dynamical mean field theory was recently successfully applied to
VO -- a material which undergoes the f amous Mott-Hubbard
metal-insulator transition upon Cr doping. The aim of this sh ort paper is to
emphasize two aspects of our recent results: (i) the filling of the
Mott-Hubbard gap with increasing temperature, and (ii) the peculiarities of the
Mott-Hubbard transition in this system which is not characterized by a diver
gence of the effective mass for the -orbital.Comment: 2 pages, 3 figures, SCES'04 conference proceeding
The cross-correlation between galaxies of different luminosities and Colors
We study the cross-correlation between galaxies of different luminosities and
colors, using a sample selected from the SDSS Dr 4. Galaxies are divided into 6
samples according to luminosity, and each of these samples is divided into red
and blue subsamples. Projected auto-correlation and cross-correlation is
estimated for these subsample. At projected separations r_p > 1\mpch, all
correlation functions are roughly parallel, although the correlation amplitude
depends systematically on luminosity and color. On r_p < 1\mpch, the auto- and
cross-correlation functions of red galaxies are significantly enhanced relative
to the corresponding power laws obtained on larger scales. Such enhancement is
absent for blue galaxies and in the cross-correlation between red and blue
galaxies. We esimate the relative bias factor on scales r > 1\mpch for each
subsample using its auto-correlation function and cross-correlation functions.
The relative bias factors obtained from different methods are similar. For blue
galaxies the luminosity-dependence of the relative bias is strong over the
luminosity range probed (-23.0<M_r < -18.0),but for red galaxies the dependence
is weaker and becomes insignificant for luminosities below L^*. To examine
whether a significant stochastic/nonlinear component exists in the bias
relation, we study the ratio R_ij= W_{ii}W_{jj}/W_{ij}^2, where W_{ij} is the
projected correlation between subsample i and j. We find that the values of
R_ij are all consistent with 1 for all-all, red-red and blue-blue samples,
however significantly larger than 1 for red-blue samples. For faint red - faint
blue samples the values of R_{ij} are as high as ~ 2 on small scales r_p < 1
\mpch and decrease with increasing r_p.Comment: 25 pages, 18 figures, Accepted for publication in Ap
Scaling properties of the redshift power spectrum: theoretical models
We report the results of an analysis of the redshift power spectrum
in three typical Cold Dark Matter (CDM) cosmological models, where
is the cosine of the angle between the wave vector and the line-of-sight.
Two distinct biased tracers derived from the primordial density peaks of
Bardeen et al. and the cluster-underweight model of Jing, Mo, & B\"orner are
considered in addition to the pure dark matter models. Based on a large set of
high resolution simulations, we have measured the redshift power spectrum for
the three tracers from the linear to the nonlinear regime. We investigate the
validity of the relation - guessed from linear theory - in the nonlinear regime
where
is the real space power spectrum, and equals . The
damping function which should generally depend on , , and
, is found to be a function of only one variable
. This scaling behavior extends into the nonlinear regime,
while can be accurately expressed as a Lorentz function - well known from
linear theory - for values . The difference between
and the pairwise velocity dispersion defined by the 3-D peculiar velocity of
the simulations (taking ) is about 15%. Therefore is a
good indicator of the pairwise velocity dispersion. The exact functional form
of depends on the cosmological model and on the bias scheme. We have given
an accurate fitting formula for the functional form of for the models
studied.Comment: accepted for publication in ApJ;24 pages with 7 figures include
Galaxy Groups in the SDSS DR4: II. halo occupation statistics
We investigate various halo occupation statistics using a large galaxy group
catalogue constructed from the SDSS DR4 with an adaptive halo-based group
finder. The conditional luminosity function (CLF) is measured separately for
all, red and blue galaxies, as well as in terms of central and satellite
galaxies. The CLFs for central and satellite galaxies can be well modelled with
a log-normal distribution and a modified Schechter form, respectively. About
85% of the central galaxies and about 80% of the satellite galaxies in halos
with masses M_h\ga 10^{14}\msunh are red galaxies. These numbers decrease to
50% and 40%, respectively, in halos with M_h \sim 10^{12}\msunh. For halos of
a given mass, the distribution of the luminosities of central galaxies, ,
has a dispersion of about 0.15 dex. The mean luminosity (stellar mass) of the
central galaxies scales with halo mass as
() for halos with masses M\gg 10^{12.5}\msunh, and
both relations are significantly steeper for less massive halos. We also
measure the luminosity (stellar mass) gap between the first and second
brightest (most massive) member galaxies, (). These gap statistics, especially in halos with M_h \la
10^{14.0}\msunh, indicate that the luminosities of central galaxies are
clearly distinct from those of their satellites. The fraction of fossil groups,
defined as those groups with , ranges from for groups with M_h\sim 10^{14}\msunh to 18-60% for groups with
M_h\sim 10^{13}\msunh. Finally, we measure the fraction of satellites, which
changes from for galaxies with \rmag\sim -22.0 to for
galaxies with \rmag\sim -17.0. (abridged)Comment: 16 pages, 11 figures. Accepted for publication in Ap
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